1. Field of the Invention:
The present invention relates to voltage regulators. More specifically, the present invention relates to digital switching voltage regulators.
While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility.
2. Description of the Related Art:
Analog and digital switching voltage regulators are known in the art. In analog switching regulators the current shunted by a switch is proportional to the difference between an existing load voltage and a desired load voltage. The current shunting switch in analog regulators is typically a transistor operating in the linear mode. In certain applications the often high power consumption of analog regulators is reduced by "tapping" a power supply at less than the available supply voltage. In this manner the voltage across the transistor switch is reduced thereby reducing power consumption. However, this reduction in the transistor operating point voltage induces a corresponding reduction in the amount of power (V.sub.TAP * I) which may be shunted by the transistor prior to saturation thereof. Hence, the regulation range of the analog regulator is limited in proportion to this reduction in power handling capability.
In contrast, conventional digital regulators are not typically so limited in current or power handling capability. Conventional digital switching regulators effect regulation within adjacent voltage "windows" centered about voltage setpoints sequentially offset from the desired load voltage. These regulators include an array of modules each having a window comparator centered about one of the setpoints. Each module typically includes a power supply (e.g., a current source) and a current shunt switch controlled by the window comparator. The array of modules is then connected in parallel with a load.
This conventional digital approach is generally expensive to implement as each switch requires an associated window comparator. Further, the voltage setpoints about which the window comparators within each module are centered are typically provided by a central amplifier. Hence, failure of this central amplifier results in a complete loss of regulation capability. This risk may be unacceptable in applications such as spacecraft load (bus) voltage regulation which require a high degree of reliability.
An alternative digital switching regulator utilizing a shift register has recently been developed which overcomes the limitations mentioned above in connection with conventional digital switching regulators. Specifically, in an article entitled "10 kW Solar Array Switching Unit Performance Test Results", published in the Proceedings of the 20th Intersociety Energy Conversion Engineering Conference in 1985, Fleck et al. disclosed a `voltage controlled solar array`. The regulator developed by Fleck includes an array of solar panels (current sources) connected to an array of shunt switches. The solar panels and shunt switches are connected in parallel with a load. When turned on, a given shunt switch diverts to ground the current from the solar panel connected thereto. Individual data registers within a shift register are operatively coupled to the shunt switches. A logic one or zero within each of the data registers turns the associated switch on or off, respectively. A comparator monitors the load voltage and compares it with a desired reference voltage to generate a logic signal which is used to control the shift register and hence the shunt switches. Data within the shift register is updated in accordance with the logic signal in response to the application of a clock signal to the shift register. In this manner a relatively constant voltage is maintained across the load despite changes in the impedance or current requirements thereof.
In the regulator developed by Fleck, the array of switches is controlled by a shift register and a single comparator. This implementation is more economical than that of the conventional digital switching regulators which have a comparator associated with each switch.
However, in the regulator of Fleck both the clock signal and the logic signal generated by the comparator are continuously applied to the shift register. As a consequence, at least one bit within the shift register will change with each clock cycle even when the instantaneous bus voltage is substantially identical to the desired bus voltage. In this manner at least one switch changes state with every clock cycle even in the presence of a constant (steady state) bus current draw. This somewhat random switching results in an unpredictable, nonuniform ripple voltage and can increase power dissipation in the shunt switches.
Further, where the ability to accommodate large changes in bus current draw is desired, a number of the regulators developed by Fleck may need to be combined in parallel. However, since one switch within each regulator changes state with each clock pulse, approximately as many switches as the number of regulators combined in parallel changes state with each clock pulse. These essentially independent switch state changes may lead to unpredictable bus voltage transients in such a parallel arrangement of regulators.
In addition, when combined in parallel the regulators developed by Fleck would generally have staggered reference voltages (setpoints) so as not to mutually interfere. These setpoints should be located as close as possible to the setpoint associated with the desired bus voltage so as to minimize bus voltage variation. However, the setpoints also need to be adequately spaced to prevent the ripple voltages from regulators with adjacent reference voltages from interacting and possibly growing to unstable proportions. As mentioned above, the amplitude of the ripple voltage within the regulators of Fleck is not well defined. Hence, in a parallel connection of the regulators developed by Fleck the regulator setpoints may not be able to be positioned sufficiently near the desired bus voltage to reduce bus voltage variations to acceptable levels for certain applications.
Hence, a need in the art exists for a digital switching voltage regulator which provides a regulated voltage having a relatively small, well defined ripple component.